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<meta name="description" content="嵌入式工程师的0x10个问题 1、用预处理指令#define声明一个常数，用以表示一年中有多少秒 1#define SECONDS_PER_YEAR (60 * 60 * 24 * 365)UL   #define语法的基本知识，不能以分号结尾 懂得预处理器将计算常数表达式的值 这个值在16位中将溢出  2、写一个标准宏MIN，这个宏输入两个参数并返回较小的一个 1#define MIN(A,B)">
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          <h2 class="post-title" itemprop="name headline">嵌入式工程师的0x10个问题</h2>
        

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        <p><strong>嵌入式工程师的0x10个问题</strong></p>
<p>1、用预处理指令#define声明一个常数，用以表示一年中有多少秒</p>
<figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">#<span class="meta-keyword">define</span> SECONDS_PER_YEAR (60 * 60 * 24 * 365)UL</span></span><br></pre></td></tr></table></figure>

<ul>
<li>#define语法的基本知识，不能以分号结尾</li>
<li>懂得预处理器将计算常数表达式的值</li>
<li>这个值在16位中将溢出</li>
</ul>
<p>2、写一个标准宏MIN，这个宏输入两个参数并返回较小的一个</p>
<figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">#<span class="meta-keyword">define</span> MIN(A,B) ((A)&lt;=(B)?(A):(B))</span></span><br></pre></td></tr></table></figure>

<ul>
<li><p>标识#define在宏中应用的基本知识。这是很重要的，因为直到嵌入(inline)操作符变为标准C的一部分，宏是方便产生嵌入代码的唯一方法，对于嵌入式系统来说，为了能达到要求的性能，嵌入代码经常是必须的方法。</p>
</li>
<li><p>三重条件操作符的知识。这个操作符存在C语言中的原因是它使得编译器能产生比if-then-else更优化的代码，了解这个用法是很重要的；</p>
</li>
<li><p>懂得在宏中小心地把参数用括号括起来</p>
</li>
<li><p>我也用这个问题开始讨论宏的副作用，例如：当你写下面的代码时会发生什么事？</p>
<figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">least = MIN(*p++, b);</span><br></pre></td></tr></table></figure>


</li>
</ul>
<p>3、用变量a给出以下定义</p>
<ul>
<li><p>​    一个整型变量</p>
<figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">int</span> a;</span><br></pre></td></tr></table></figure>
</li>
<li><p>​    一个指向整型数的指针</p>
<figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">int</span> *a;</span><br></pre></td></tr></table></figure>
</li>
<li><p>一个指向指针的指针，它的指针是指向一个整型数</p>
<figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">int</span> **a;</span><br></pre></td></tr></table></figure>
</li>
<li><p>一个10个整型数的数组</p>
<figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">int</span> a[<span class="number">10</span>];</span><br></pre></td></tr></table></figure>
</li>
<li><p>一个有10个指针的数组，该指针是指向一个整型数</p>
<figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">int</span> *a[<span class="number">10</span>];</span><br></pre></td></tr></table></figure>
</li>
<li><p>一个指向有10个整型数数组的指针</p>
<figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">int</span> (*a)[<span class="number">10</span>];</span><br></pre></td></tr></table></figure>
</li>
<li><p>一个指向函数的指针，该函数有一个整型参数并返回一个整数</p>
<figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">int</span> (*a)(<span class="keyword">int</span>);</span><br></pre></td></tr></table></figure>
</li>
<li><p>一个有10个指针的数组，该指针指向一个函数，该函数有一个整型参数并返回一个整数</p>
<figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">int</span> (*a[<span class="number">10</span>])(<span class="keyword">int</span>);</span><br></pre></td></tr></table></figure>

</li>
</ul>
<p>4、static关键字有什么用？</p>
<ul>
<li>在函数内，一个被声明为静态的变量在这一涵数被调用过程中维持其值不变</li>
<li>在模块内(但在函数体外)，一个被声明为静态的变量可以被模块内所有函数所访问，不能被模块外其它函数访问，模块内全局变量。</li>
<li>在模块内用作函数的定义，声明该函数为静态函数，只可被该模块内函数调用。</li>
</ul>
<p>5、const关键字有什么用？</p>
<ul>
<li><p>只读、常数</p>
<figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">const</span> <span class="keyword">int</span> a;<span class="comment">//一个常整型数</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">int</span> <span class="keyword">const</span> a;<span class="comment">//一个常整型数</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">const</span> <span class="keyword">int</span> *a;<span class="comment">//指向常整型数的指针</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">int</span> * <span class="keyword">const</span> a;<span class="comment">//指向整型数的常指针</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">int</span> <span class="keyword">const</span> * a <span class="keyword">const</span>;<span class="comment">//一个指向常整型数的常指针</span></span><br></pre></td></tr></table></figure>

</li>
</ul>
<p>6、volatile关键字有什么用？</p>
<p>​    一个定义为volatile的变量表明该变量可能会意向不到的被改变，处理器每次通过地址读取该值而不是通过寄存器的拷贝值。</p>
<p><em>常用在：</em></p>
<ul>
<li>并行设备的硬件寄存器</li>
<li>中断服务子程序中会访问到的非自动变量</li>
<li>多线程中被多个任务共享的变量</li>
</ul>
<p>7、给定一个整型变量a，设置a的bit3，清除a的bit3</p>
<figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">unsigned</span> <span class="keyword">int</span> a;</span><br><span class="line"></span><br><span class="line">a|=(<span class="number">0x1</span>&lt;&lt;<span class="number">3</span>);</span><br><span class="line"></span><br><span class="line">a&amp;=~(<span class="number">0x1</span>&lt;&lt;<span class="number">3</span>);</span><br></pre></td></tr></table></figure>

<p>8、要求设置一绝对地址为0x67a9的整型变量的值为0xaa66，编译器是一个存粹的ANSI编译器</p>
<figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">int</span> *p=(<span class="keyword">int</span>*)<span class="number">0x67a9</span>;</span><br><span class="line">p[<span class="number">0</span>]=<span class="number">0xaa66</span>;<span class="comment">//或*p=0xaa66;//*((int*)0x67a9)=0xaa66;</span></span><br></pre></td></tr></table></figure>
      
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